My grease pencil colours go different colours in render mode
I would hazard a guess that you've filled the character using vertex colour instead of material colour. Blender automatically sets the default vertex colour to that delightful shade of green you're seeing.
Vertex colour is only visible in Material Preview and Rendered view so it's likely you were in Solid view and may have set this accidently?
If you're not sure what I'm talking about,  if you look at your first screenshot about midway at the top of the tool panel you'll see the offending green colour as the selected Vertex colour material. The settings pictured suggest the tool is set up to use material colour (the globe Icon) but it's probably likely when filling it was set the vertex colour?
To remedy you could go through and use vertex paint mode to recolour every point or just refill making sure the mode is set to material (globe turned on!).
Great drawing by the way! I hope that resolves your problem.
Matt